What species, with what species?
First we need to identify whether or not this is a territorial driven behavior, or a jealous bird behavior, or a dominance issue...
How you deal with it depends on figuring out the root cause.
Zons in particular, especially if you have two males of breeding age in the house, sometimes lay claim to a particular territory, and if another bird violates it, WATCH OUT... it's the equivalent to "Defend the nest from intruders." Give them each a spot. The bird defending his territory gets to keep it, and likely won't be interested in taking the other bird's spot. End of conflict. That's yours, this is yours. Big bird has spoken...
Sometimes it's attention driven. That OTHER bird is taking mom's attention away from me. I'll teach you not to do that... Increasing 1 on 1 time with the offended bird, and teaching that bird he/she has to share, but you will get your turn... (just like a toddler!)
Sometimes, it's a bird with dominance issues. I can kick your butt, therefore I will. In that case you simply keep them separated, and the "bully bird" gets caged and covered when he gets out of line. (Again, just like a toddler!)
